Calculate wavelengths and apply oscillating-charge and antenna-polarisation rules.
Method and worked examples
An oscillating charge emits electromagnetic radiation at its oscillation frequency. The wave contains electric and magnetic fields perpendicular to each other and to its direction of travel. In vacuum, c=fλ, so a higher frequency means a shorter wavelength. Electromagnetic waves need no material medium. An incident wave can drive charges in a receiving antenna to oscillate at the wave frequency.
The electric-field oscillation direction defines linear polarisation. A straight receiving antenna responds most strongly when aligned with that electric field and ideally minimally when perpendicular to it. The propagation direction is a separate perpendicular direction. Radio, microwave, visible and X−ray radiation all obey the vacuum wave-speed relationship despite their different frequencies and uses.
Example 1
A transmitter operates at 100 MHz. Use c=3.00×108ms−1. Find wavelength.
f=1.00×108 Hz. λ=fc=3.00m.
Example 2
A wave travels east with its electric field oscillating vertically. How should a straight receiving antenna be aligned?
Vertically, parallel to the electric-field oscillation, for the strongest ideal response.
Watch out:Electromagnetic waves propagate through vacuum; sound requires a material medium.
1. Electromagnetic waves and polarisation | Year 12 Physics | Ascent Arcade